The present invention relates generally to industrial process transmitters used in industrial process control systems. More particularly the present invention relates to flame arrestors for industrial process transmitters.
Process instruments are used to monitor process parameters, such as pressure, temperature, flow and level, of process fluids used in industrial processes. For example, process transmitters are typically employed in industrial manufacturing facilities at multiple locations to monitor a variety of process parameters along various production lines. Process transmitters include sensors that produce an electrical output in response to physical changes in the process parameter. For example, pressure transmitters include pressure transducers that produce an electrical output as a function of the pressure of a process fluid, such as in water lines, chemical tanks or the like. Each process transmitter also includes transmitter electronics for receiving and processing the electrical output of the sensor so that the transmitter and process parameter can be monitored locally or remotely. Locally monitored transmitters include displays, such as LCD screens, that show the electrical output at the site of the process transmitter. Remotely monitored transmitters include electronics that transmit the electrical output over a control loop or network to a central monitoring location such as a control room. Configured as such, the process parameter can be regulated from the control room by including automated switches, valves, pumps and other similar components in the process control system and the control loop.
Sensors within the transmitter respond to a physical change in the process fluid being monitored. For example, pressure sensors used in pressure transmitters include a flexible sensor element such as a diaphragm or capacitor plate. The sensor element is typically connected to the process fluid through a simple hydraulic system that communicates the process fluid pressure to the sensor. The hydraulic system comprises a sealed passageway in which the sensor element is positioned at a first end, and a flexible isolation diaphragm is positioned at a second end to engage the process fluid. The sealed passageway is filled with a precise amount of hydraulic fluid that adjusts the position of the sensor element as the process fluid influences the isolation diaphragm. As the pressure of the process fluid changes, the position of the sensor element changes, resulting in a proportional change in a pressure sensor signal.
Industrial process transmitters are often used in facilities with environments where various gases or compounds that have the potential to ignite are present. For example, transmitters are often operated in natural gas processing plants or facilities where hydrogen gas is used. Alternatively, industrial process transmitters may be used to monitor process fluids which are themselves combustible. In order to ensure overall safe operation of the facility, industrial process transmitters are certified to be explosion-proof or flame-proof to prevent flames from being able to pass through the transmitter. For example, electronics within the transmitter may cause a spark that has the potential to ignite a gas in the environment of the facility or a process fluid in the case of a ruptured hydraulic passageway. Thus, it is desirable to ensure that all passageways connecting the interior of the transmitter to the exterior environment be configured to extinguish or quench flames traveling through the passageways.
Conventional configurations for flame-proofing sealed hydraulic passageways, such as described in U.S. Pat. No. 4,970,898 to Walish et al. and assigned to Rosemount Inc., Eden Prairie, Minn., comprise machining a long and narrow cylindrical path through a transmitter component, such as a flange or a plug. The flange or plug typically includes a slab of stainless steel having a thickness commensurate with the appropriate length required to provide the flame-proof path. Flame-proof paths are typically a function of the length to diameter (L/D) ratio of the passageway, as well as the ratio of the perimeter to area (P/A) of the passageway. Adequate L/D ratios, however, typically lengthen the thickness of the flange or plug over what would typically be required for structural purposes. Furthermore, in order to machine such a small diameter through such a thick slab, electrical discharge machining (EDM) is often used. EDM operations, however, produce a slag layer, which comprises a layer of material lining the hole consisting of re-cast material of the slab. The slag is dirty and difficult to clean. The slag also harbors moisture, which may interfere with operation of the pressure transmitter and hydraulic system by, for example, altering the dielectric of the sensor. EDM processes are also expensive relative to conventional machining processes, such as drilling operations. There is, therefore, a need for improved flame-proof paths in industrial process transmitters, particularly those that are cleaner, smaller and less expensive.